1. Field of the Invention
The present invention relates to an apparatus for reproducing a video signal recorded in a recoding medium such as a magnetic tape or the like by a helical scanning system, in which a rotating two head system is adopted together with a DTF system in a head portion and the so-called pilot signal system is employed for tracking control. Particularly, the present invention relates to a video signal reproducing apparatus in which an improvement is made in still reproduction control so that noise at the time of still reproduction can be diminished.
2. Description of the Prior Art
An example of a tracking control method using the so-called pilot signal system in which positioning of reproducing heads is controlled based on pilot signals recorded in an overlapping manner with a video signal is proposed in U.S. Pat. No. 4,297,733. In such a pilot signal system, for example, four kinds of tracking pilot signals f.sub.1, f.sub.2, f.sub.3 and f.sub.4 having different frequencies are applied. The pilot signals f.sub.1 to f.sub.4 are selected by a field period and recorded in an overlapping manner with a video signal. As for the frequencies of the pilot signals, the frequency of the first signal f.sub.1 is 102.544 kHz, that of the second signal f.sub.2 is 118.951 kHz, that of the third signal f.sub.3 is 165.210 kHz and that of the fourth signal f.sub.4 is 148.689 kHz, for example. These pilot signals are recorded in a specified order in which a difference between the frequency of a pilot signal on one video track and that of a pilot signal on one adjacent track is approximately 16 kHz and a difference between the frequency of the above mentioned pilot signal and that of a pilot signal on the other adjacent track is approximately 46 kHz.
Using these pilot signals, tracking control for reproduction is performed in the below described manner. When a reproduced track is scanned by a reproducing head, the pilot signals recorded on the left and right tracks adjacent to the scanned track are also read to some extent due to a side-read effect of the reproducing head. For example, if the pilot signal f.sub.1 is recorded on the reproduced track, the pilot signals f.sub.4 and f.sub.2 recorded on both adjacent tracks are also read out besides the pilot signal f.sub.1. Then, a reference signal f.sub.1 having a frequency equivalent to that of the tracking pilot signal f.sub.1 on the reproduced track is fed into the pilot signals thus read out. As a result, beats are generated between the crosstalk components f.sub.4 and f.sub.2 from the adjacent tracks and the reference signal f.sub.1. Accordingly, if these beats are extracted and a comparison is made between the level of a beat component of 46 kHz and the level of a beat component of 16 kHz, tracking error can be detected. More specifically, in case of normal tracking of the reproducing head, the output levels of the pilot signals read out from the right and left tracks are equal and accordingly the beat component of 46 kHz and the beat component of 16 kHz generated respectively between the reproduced pilot signals and the injected reference signal have the same level. On the other hand, in case where the reproducing head is closer to either of the adjacent tracks, the levels of the above stated beat components are not equal. As a result, a tracking error can be detected. Thus, based on the detection, servo control of a tape sending system such as a capstan is performed.
U.S. Pat. No. 4,106,065 discloses an example of the so-called DTF system in which positioning of a magnetic head is controlled by a control signal with the magnetic head being attached to a bimorph or a piezoelectric bender or the like.
Such DTF system is a control system which is particularly utilized for special modes of reproduction such as the so-called cue reproduction (forward fast reproduction for searching), rewinding reproduction and the like. In case of such a special mode of reproduction, as is different from the ordinary mode of reproduction, scanning of a reproducing heads is not applied only along a desired recorded track, but is also applied in a manner crossing a plurality of recorded tracks. As a result, the scanning locus of the reproducing heads cannot be made to coincide with the recorded tracks only by the control of the tape sending system and the like. For this reason, by means of a control mechanism of the DTF system, control is made such that the reproducing heads are moved in the transversal direction of the tracks based on an error signal obtained as a mixed signal of the reproduced pilot signals and the reference signal, whereby the reproducing heads scan accurately the recorded tracks.
However, at the time of still reproduction, which is one of the special forms of reproduction, travelling of the magnetic tape is completely stopped and scanning of the reproducing heads are applied in a manner in which the reproducing heads cross a plurality of specified recorded tracks. As a result, in such a conventional reproducing apparatus, a plurality of pilot signals are reproduced by the reproducing heads and disadvantages are involved that it is difficult and takes much time to determine which signal out of those pilot signals has frequency coincident with the frequency of the reference signals to be supplied in order to obtain an error signal most suitable for the DTF control of the reproducing heads.
A more concrete description will be made in the following. FIG. 1 is a block diagram showing an example of an error signal forming circuit for providing an error signal to two reproducing heads of a rotating two head system, including a mechanism for the DTF system (these two reproducing heads being referred to hereinafter as "movable heads" including the DTF mechanism; In case where this invention is applied to a video signal recording and reproducing apparatus, the movable heads are adapted to have a double function of recording and reproduction). Referring to FIG. 1, signals reproduced by both movable heads are supplied from a reproduced signal input terminal 1 to a filter 2. Reproduced pilot signals are extracted from the reproduced signals through the filter 2 and the extracted reproduced pilot signals are supplied to a mixer 3. On the other hand, through a control signal input terminal 4, a 2-bit control signal is applied to a reference signal generating circuit 5. The reference signal generating circuit 5 selects successively reference signals f.sub.1 to f.sub.4 in response to the control signal and supplies the reference signals to the mixer 3. The mixer 3 mixes the successively selected reproduced pilot signals and reference signals and supplies the mixed signals to a bandpass filter 6 of 16 kHz and to a bandpass filter 7 of 46 kHz, respectively. The signal components extracted by the bandpass filters 6 and 7 are detected by AM detectors 8 and 9, respectively, so as to be compared by a comparator 10. The output of the comparator 10 is supplied, on one hand, directly to a switch 11 and on the other hand to the switch 11 through an inverter 12. The switch 11 is turned to either side alternately in response to the RF pulse for head selection applied from a head selection signal input terminal 13. Consequently, an error signal for DTF control and tracking control is provided from an error signal output terminal 14.
It is for the below described reasons that the polarity of the error signal is thus inverted in response to the selection of a head. In one of the reproducing heads, a signal component of 46 kHz for example increases when the scanning position deviates in the tape travelling direction, and a signal component of 16 kHz increases when the tracing position deviates in the opposite direction. On the contrary, in the other reproducing head, a signal component of 16 kHz increases when the scanning position deviates in the tape travelling direction, and a signal component of 46 kHz increases when the scanning position deviates in the opposite direction. Accordingly, by inverting the polarity of the error signal for each head, it can be automatically determined that when a signal component of 46 kHz increases in the error signal, the scanning position of the head deviates in the tape travelling direction, and that the scanning position of the head deviates in the direction opposite to the tape travelling direction when a signal component of 16 kHz increases.
In the following, description will be made of a conventional method of control of the above described error signal forming circuit in the still reproduction mode.
Let us assume that in the ordinary reproduction mode, one of the two movable heads scans the tracks where the pilot signals having frequencies f.sub.1 and f.sub.3 are recorded, that the other movable head scans the tracks where the pilot signals having frequencies f.sub.2 and f.sub.4 are recorded, and that travelling of the tape is stopped in response to the instruction of still reproduction. Then, the control signal applied from the control signal input terminal 4 changes and, as a result, the reference signal generating circuit 5 operates in the following manner for the first several frames. Specifically stated, a reference signal having frequency f.sub.1 is provided from the generating circuit 5 to the mixer 3 while one of the movable head scans the tracks where the pilot signals f.sub.1 or f.sub.3 are recorded and a reference signal having frequency f.sub.2 is provided from the generating circuit 5 to the mixer 3 while the other movable head scans the tracks where the pilot signals f.sub.2 or f.sub.4 are recorded.
Consequently, in the period of the first several frames after the start of still reproduction, the pilot signal f.sub.1 or f.sub.3 reproduced by one movable head is mixed with the reference signal f.sub.1 by means of the mixer 3 and the pilot signal f.sub.2 or f.sub.4 is mixed with the reference signal f.sub.2 by means of the mixer 3.
An output signal of the mixer 3 is supplied to the AM detecting circuit 8 through the 16 kHz bandpass filter 6, in the same manner as described above, as well as to the AM detecting circuit 9 through the 46 kHz bandpass filter 7. Then, by means of the comparator 10, the outputs of both detecting circuits 8 and 9 are compared to generate an error signal.
As this time, if one movable head scans the track where the pilot signal f.sub.1 is recorded and the other movable head scans the the track where the pilot signal f.sub.2 is recorded, the output signal of the mixer 3 becomes 0 and the error signal to be described below also becomes 0.
However, since the travelling of the tape is stopped at the time of still reproduction, the scanning loci of both movable heads maintained in the ordinary reproduction positions deviate from the tracks where the signals are recorded.
FIG. 2 illustrates an example in which scanning loci and recorded tracks do not coincide. At the time of still reproduction, as shown in FIG. 2 for example, the movable heads scan slantingly the tracks 17, 18 and 19 where the pilot signals f.sub.3, f.sub.4 and f.sub.1 are recorded. Accordingly, the scanning loci are shown as a double hatched region denoted by the reference numeral 20. The reference numerals 15 and 16 in FIG. 2 denote tracks where the pilot signals f.sub.1 and f.sub.2 are recorded, respectively. The reference numeral 21 in FIG. 2 denotes a magnetic tape.
Thus, the pilot signals reproduced by both movable heads comprise the pilot signals having frequencies f.sub.3, f.sub.4 and f.sub.1, respectively.
Accordingly, the output signal of the mixer 3 based on the pilot signals f.sub.3, f.sub.4 and f.sub.1 reproduced by one movable head and the reference signal f.sub.1 becomes a beat signal having a frequency equivalent to the difference between the reproduced pilot signal f.sub.4 and the reference signal f.sub.1, namely, 46 kHz, and the output signal of the mixer 3 based on the pilot signals f.sub.3, f.sub.4 and f.sub.1 reproduced by the other movable head and the reference signal f.sub.2 becomes a beat signal having a frequency equivalent to the difference between the reproduced pilot signal f.sub.3 and the reference signal f.sub.2, namely, 46 kHz. The reason is that if the frequency of a reproduced pilot signal and the frequency of a reference signal are equal, or if the frequency difference between both signals is other than 16 kHz or 46 kHz (caused by f.sub.3 and f.sub.1 or F.sub.4 and f.sub.2), the beat signals are removed through the bandpass filters 6 and 7.
As a result, the level of the detected signal provided from the detecting circuit 8 becomes approximately 0 and the level of the detected signal of 46 kHz provided from the detecting circuit 9 increases. Accordingly, the output signal of the comparator 10 which compares both detected signals is a negative output signal proportional to the difference of levels of the two detected signals. Subsequently, in the same manner as described above, an output directly provided from the comparator 10 and an output obtained from the inverter 12 which inverts the output of the comparator 10 are selected alternately by the switch 11 for each field, so that an error signal turning positive and negative consecutively can be obtained. With a positive error signal, one movable head is controlled and with a negative error signal, the other movable head is controlled. Thus, based on both error signals, the positioning of both movable heads is controlled. More specifically, based on the level of the error signal, the position of the movable heads are moved, so that the level of the error signal may be decreased.
The level of the components of the frequencies f.sub.3 and f.sub.4 of the pilot signals reproduced by the two movable heads change all the time according to the scanning of the movable heads.
However, even if the reference signals f.sub.1 and f.sub.2 continue to be applied, the scanning locus 20 of the heads deviates considerably from the tracks 19 and 16 where the pilot signals f.sub.1 and f.sub.2 are recorded. As a result, the positioning control of the movable heads based on the error signal cannot be made satisfactorily. Accordingly, when the reference signals f.sub.1 and f.sub.2 are applied, an error signal continues to be supplied from the output terminal 14 since the positioning the movable heads cannot be controlled adequately.
Then, for the subsequent several frames, the frequencies of the reference signals provided from the generating circuit 5 to the mixer circuit 3 are changed to f.sub.1 and f.sub.4.
If an error signal still continues to be supplied after the change of the frequencies of the reference signals to f.sub.1 and f.sub.4, the frequencies of the reference signals provided from the generating circuit 5 to the mixer circuit 3 are changed to f.sub.3 and f.sub.4 in a period of the further subsequent several frames.
Then, in the case of FIG. 2, the movable heads are controlled to be placed in the most suitable positions when the frequencies of the reference signals are changed to f.sub.3 and f.sub.4.
FIG. 3 shows a state where the movable heads are controlled to be in the most suitable positions. As shown in FIG. 3, one movable head (the scanning locus thereof being denoted by the reference numeral 22) scans the track 17 where the pilot signal of the frequency f.sub.3 is recorded, and the other movable head (the scanning locus thereof being denoted by the reference numeral 23) scans the track 18 where the pilot signal of the frequency f.sub.4 is recorded, the level of the error signal at this time being approximately 0.
When the level of the error signal becomes almost 0, the generating circuit 5 stops the change of the frequencies of the reference signals and then, noiseless still reproduction is made.
However, as is clearly understood from the foregoing description, such a conventional control method involves disadvantages that it takes much time to control the positioning of the movable heads most suitably and noiseless still reproduction cannot be made promptly.